Since a UV light emitting diode emits light having a relatively short peak wavelength (generally, a peak wavelength of 400 nm or less), a light emitting region is formed of AlGaN containing 10% or more of Al in fabrication of a UV light emitting diode using a nitride semiconductor. In such a UV light emitting diode, if n-type and p-type nitride semiconductor layers have smaller energy band-gaps than energy of UV light emitted from an active layer, UV light emitted from the active layer can be absorbed into the n-type and p-type nitride semiconductor layers in the light emitting diode. Then, the light emitting diode suffers from significant deterioration in luminous efficacy. Accordingly, not only the active layer of the UV light emitting diode, but also other semiconductor layers placed in a light emitting direction of the UV light emitting diode, particularly, an n-type semiconductor layer, contain 10% or more of Al.
In fabrication of a UV light emitting diode, a sapphire substrate is generally used as a growth substrate. However, when AlxGa(1-x)N layer (0.1≤x≤1) containing 10% or more of Al is grown on the sapphire substrate, the UV light emitting diode suffers from cracking or breaking caused by thermal or structural deformation due to a high Al composition ratio. This problem is caused by lattice mismatch or a difference in coefficient of thermal expansion between the sapphire substrate and the AlxGa(1-x)N layer (0.1≤x≤1). Specifically, due to a difference in coefficient of thermal expansion between the sapphire substrate having a relatively high coefficient of thermal expansion and a nitride semiconductor having a relatively low coefficient of thermal expansion, a wafer suffers from bowing into a concave shape upon growth of the nitride semiconductor at high temperature (at about 1000° C. or more). When the growth temperature is decreased again, the wafer is flattened again or bowed into a convex shape. Due to bowing of the wafer, tensile stress gradually increases in an upward direction of the wafer, thereby causing generation of cracks in the nitride semiconductor. Such cracks cause deterioration in production yield and quality of light emitting diodes.